1 | 54 communication systems 8 th lecture chair of communication systems department of applied...

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Communication Systems8th lecture

Chair of Communication SystemsDepartment of Applied Sciences

University of Freiburg2006

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Communication SystemsAdministrative stuff

● Lecture on 13.06 might be called off. Please check the webpage before the lecture.

● 06.06, 08.06, 15.06 are holidays, no lecture, no practical.

● Next practical course is on 22.06, in RZ basement -101.

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Communication SystemsLast lectures

● We started with rather modern communication technologies and introduced the Internet Protocol as a global orientated packet switching network technology– IP can be run over very different physical media and

intermediate protocols

– And IP is used for more and more networked services

– Very popular traditional service is telephony mostly 1:1 voice communication

– With the upcoming “Voice-over-IP” we could observe a merge of both networks

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Communication SystemsUpcoming lectures

● To get an idea how traditional and modern wireless telephony networks work, we give an introduction to ISDN, GSM and UMTS

● First traditional telephony networks its history and their concepts in general

● Digitization of voice - PCM● Then introduction to ISDN – a completely digitalized

communication infrastructure– call setup and global routing in telephony networks

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Communication Systemsplan for this lecture

● History of telephony networks and wireless information networks● Line switching● DTMF – dual tone multi frequency● Telephony protocol● Standards in telecommunication● Digital telephony networks – PCM● ISDN – Integrated Services Digital Network● D channel● DSS1 layer 3 protocol

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Communication SystemsHistory of telephony networks

● Traditional analogous telephony networks– 1848: State Telegraphy System in Prussia (Siemens)

– 1851: First trans-sea cable between Dover and Calais

– 1858: Transatlantic line-based telegraphy between Europe and America

– 1866: Durable transatlantic cable

– 1876: Bell patents the “phone” (Reiss in Germany)

– 1880: 50.000 participants in US phone network

– 1881: Berlin opens the first “Fernsprechamt”

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Communication SystemsHistory of wireless information networks

● Wireless signal transmission– Morse codes transmitted by radio (Marconi)

– 1901: Radio-based telegraphy between Europe and the US

– 1914: Introducing the teletype/telex system

– 1915: Wireless telephony NY – San Francisco

– 1920: First public radio transmission in Königs-Wusterhausen

– 1923: Start of entertaining radio in Berlin

– 1929: First radio-based TV transmission (Funkausstellung in Berlin)

– 1935: First regular public TV transmissions in Berlin

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Communication Systemsdevelopment of telephony equipment

● Traditional analogous telephony networks provides most of the standards (partly) in use up to now – Bi-directional voice channel

– Bandwidth to carry voice around 300Hz - 3,4kHz – just the characteristics of the end user devices and their microphones and earpieces

– You could hook up the old mid-thirties or sixties telephone set to your wall socket of your telephony provider or your private

telephone installation

– End devices are power supplied by the telephone exchange, so the devices independent of local sources

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Communication Systemsdevelopment of telephony equipment

● Local loop – connection of the end uses device to the telephony exchange– Device is without power when hook on cradle

– Call information is signalled with 65V alternating current

– When off-hook power supplied at around 60V by a current of 20 – 40mA

– Dial plate cuts the local loop for well defined periods to indicate dial information (~60ms cut, ~40ms closed in between – try to dial via cradle – system is rather robust in detection :-))

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Communication Systemsline switching

● End systems has to be connected somehow to each other– In the early beginnings manual switch boards (you know the

pictures of old films with young ladies called operators plugging wires to connect subscribers :-))

– around 1974

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● Switch boards– first direct-dial switch boards

appeared around 1900 used in local area nets first and from around 1920 for long distance calls – dial plates (digits 1 – 9, 0) where added to the telephone device

– using special relay boards with contacts for each dialed digit

– system operated directly controlled until around 1960s

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Communication Systemsline switching and signalling

● Early phones used a hand generator to signal assistance by the operator at the switch board

● Now: Identification of each end device through numerical ID composed of digits from decade system– dial plates (digits 1 – 9, 0) where added to the telephone device

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Communication Systemsautomated line switching

● Switch boards - routers in the telephony world– major drawbacks of this concept

– route of the call is fixed● every dialed digit switches the next relay in the switching

network● the (long distance) line was already occupied during call

setup● Next step was introduction of indirectly operated switching

networks middle of the fifties– Before routing setup the dial information was collected and

then processed

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● Analogous electronic switching networks appeared with the beginning of the 1970s– allowed new type of dial indication

● DTMF – dual tone multi frequency was introduced for dial information– Inband signalling

– pulse dial information has to be transported via copper wires and require rather high currents

– puls dialing impossible over very long distances (resistor capacity of wire) and wireless transmission

– major speedup for dialing

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Communication SystemsDTMF

– voice frequency band to the call switching center – frequencies selected in a way that no clash with “normal” voice

– multifrequency shift keying (MFSK)

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Communication SystemsDTMF signalling

– Still in use on analogous lines and for signalling e.g. on voice menu systems – digital equipment uses out-of-band

– Special codes for signalling other data (e.g. Pay card identification) and for cost signalling between switching centres

– Some people were able to produce the needed frequencies to switch off payment or setup special connections (no cost, used by Telcos for maintenance)

– “Hacking/Cracking” started not with computer networks but with automated telephony equipment – challenge of the 70s was to setup routes around the globe to call someone other in the same city (and enjoy the delay because of the huge distances)

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Communication Systemstelephony protocol

● Key dials were introduced to telephones – special optimized layout (in contradiction to keyboard layout used today)

● So we have a well known “protocol of analogous telephony connection”

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Communication SystemsProtocol of analogous telephony connection

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Communication Systemsstandards in telecommunication

● But in telephony world mostly not talked on “protocols” but interfaces

● Interfaces are well-defined connection points where different parts of the infrastructure/equipment talk to each other in a certain way

● International standardization body is ITU (International Telecommunication Union www.itu.int)

● Process of standardization completely different to the workflows in Internet bodies– No bottom up, but top down decisions

– Exclusive club of the big (state monopoly) Telcos

– High annual fees

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● Because of the old (nation state) monopolies there are many differences within the several networks– Numbering schemes

– Acoustical indication of dial states (busy, line-free, ...)

– Different use, assignment of the (wireless) frequency spectrum

– Not really compatible equipment (branch exchanges, ...) - every firm tries to use their own subset of “standards”

● With the introduction of digital networks (ISDN and mobile) agreement on global standards started

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● Inter connecting of voice streams has lots of technical problems

● Up to 1980s computerized switching centers but analogous voice connections– Fault-prone to jamming and noise

– Regeneration means amplification of noise too

● Allow data connections over telephony networks● Next step: Fully computerized switching centers

– out of band signaling of call setup

– digital voice streams allow better/perfect regeneration

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Communication Systemsdigital telephony networks - PCM

● Analogous signal– Continuous in time and value

domain

– Characterized by amplitude (signal strength) and frequency

– Bandwidth in telephony networks 300Hz - 3,4kHz

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● Sampling of a signal– Rate at least twice the max

frequency of analogous signal (Nyquest theorem)

– 2* fmaxb = 2*3,4kHz = 6,8kHz

– Internationally the sample frequency was agreed on fSample=8kHz=8000Hz=8000/s

– We get a sample period of T=1/f=1/8000=125µs

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● Analogous signal– Continuous in value domain

– Has to be translated into discrete values

● A/D convertor quantizes the signal– Splitting the value domain into

equal intervals

– Every measured value is approximated and assigned to one of the defined intervals

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● PCM defines 128 different levels for positive and 128 negative amplitude of the signal

● thus resolution is 256 bit● Sample rate is 8000 per

second● so we get 8000 Byte per

second and a bit stream of 64kbit/s

● So we have the B channel bandwidth for ISDN ...

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Communication SystemsISDN – Integrated Services Digital Network

● The development of digital switching networks led to standardization and integration of additional services into the same network– three virtual multiplex channels over the same two wire

infrastructure

– digital telephony (two independent lines on basic rate interface)

– fax, telex

– video telephony (H.323 devices may use ISDN as transport layer for their applications)

– data communication of 64 or 128kbit/s

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● Prerequisite for ISDN was digitalized infrastructure● The ISDN standard was defined in the early 1980s by the

ITU– several national standards evolved, 1TR6 in Germany, NI-1/2

in United States, DACS in UK, ...

– DSS1 is the “EURO-ISDN” used in many other countries too available from 1993

– EURO ISDN was defined by the new founded ETSI (European Telecommunication Standards Institute in 1988)

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● ISDN is commonly used in all European countries since 2000– all switching centers use ISDN backends

– so called “analogous” telephony devices (POTS – plain old telephony service) are converted to digital service at the local switching center

– 50% of the European BRI connections are in Germany

– Germany has a 30% worldwide share

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Communication SystemsISDN – and the OSI protocol stack (mostly D channel)

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Communication SystemsISDN – Basic Rate Interface

● BRI provides a total data rate of 160kbit/s– standard end user connection

– 2 B channels (“bearer” - for data, digitized voice, ...) of 64kbit/s each

– 1 D channel (data channel for out-of-band signaling) of 16kbit/s

– synchronization of 16kbit/s

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● Physical layer specifications of the Uk0– Operates over two-wire cable up to 5 km (depending on cable

diameter and quality)

– Switching center provides a 90V current to power the NTBA and one device (emergency function – to be independent on local power supply for at least one telephone)

– Other physical layer specifications for alternate U interfaces

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● BRI network termination is defined by the Uk0 interface– a special encoding (4B3T) is used: 4 bit digital to 3 baud

ternary

– 4B3T is a "block code" that uses Return-to-Zero states

– allows reduction of symbol rate to 120 kBaud (¾th) and thus distances up to 8km

– reduction of low frequencies in the signal spectrum

– better detection of code errors

– three states: negative pulse, no pulse, positive pulse

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● next state (S1 - S4) to be transmitted is indicated in column labled Go

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● Alternate encoding: 2B1Q – 2 bit digital to 1 baud quaternary representation

● 2B1Q transmission can be simply described as an amplitude modulation scheme for DC pulses

● Ordering of data blocks depends on the encoding used

Bits Voltage00 -2,5001 -0,8310 2,5011 0,83

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Communication SystemsUk0 – bit streams from switching center to NTBA

● Each frame consists of 120 ternary steps– 2*B+1*D takes 108 steps in 4 ternary blocks (tb) with 27 steps

each

– Sync channel occupies 11 steps and a “maintenance” channel (mc) 1 step

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Communication SystemsUk0 – bit streams from NTBA to switching center

● Connection is full-duplex over the two wires– Echo compensation and terminating set is needed

– NTBA splits the data streams to separate up and down onto the S0 bus

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● Instead of the traditional wall socket a NTBA (network terminal base adapter) is needed at end users site

● NTBA provides the S0 bus to which end user devices are

connected– Unidirectional – on pair of wires for each direction

– Allows up to 12 wall sockets, 8 ISDN devices (or analogous devices via a/b converter)

– Provides device power up to 4,5W

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Communication SystemsISDN – S0

● Provides the same B and D channels as Uk0– Maintains the step and octet frequency

– Handles the device plugging and device activation, deactivation

– Has to be terminates with resistors of 110 Ohm

– Uses modified AMI code with currents of -0,75 and 0,75V

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Communication SystemsS0 – AMI code

● Modified AMI code (avoid long sequences of symbols of the same type)

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Communication Systemsdata link layer for the D channel

● No distinct layering for B channels – PCM or data directly put into frames as shown on previous slides

● LAPD – Link Access Procedure on D channel– Derived from High-Level Data Link Control Protokoll (HDLC)

– Broadcasts only for network termination device

● D2 frame margin – octet of binary pattern: 01111110● Keeping of frame sequence● Error discovery● Multiplexing of more than one logical D2 connections● Flow control

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Communication Systemshigher layer protocols for the D channel

● ITU Recommendation Q.921

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Communication Systemslayer 2 for the D channel

● Flag– character is part of the Header information, hexadecimal 7E

● Address is two bytes (octets) long, and consists of three fields– Service Access Point Identifier (SAPI)

– Command/Response (C/R) bit

– Terminal Endpoint Identifier (TEI)

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Communication Systemslayer 2 for the D channel

● Control one or two octets (bytes) in length, indicates one of three frame formats– Information

– Supervisory

– Unnumbered

● Information carries Layer 3 Call Control (Q.931) data– It may carry Unnumbered Information data (TEI assignment)

or XID (Connection Management/parameter negotiation) information

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● Protocol handles the TEI (Terminal Endpoint Identifier) allocation– All devices on S0 using the same bus and have to be

addressable

– TEI assignment is started by the connected devices after successful initialization of physical layer synchronization

– Non automatic assignment uses ID0 – 63, automatic 64 – 126

– There is a special group TEI 127

● Protocol elements– information lowermost bit is set to 0

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● Protocol elements– Receive Ready - (01)

– Set Asyncronous Balance Mode Extended - (6F/7F)

– Unnumbered Information - (03)

– Disconnect - (43/53)

– Unnumbered Acknowledgement – (63/73)

● Flow control uses sequence numbers for sending and receiving

00:E1:04:00:...● Octets #4 for sending and #5 for receiving in the

information frame

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Communication Systemsdata link layer for the D channel error detection

● D channel protocol uses rather sophisticated error detection protocol

● Generates frame checksums● Generator polynom

g(x) = (x +1)(x15+x14+x13+x12+x4+x2+x +1)

g(x) = x16+x12+x5+1● 16 bit frame checksum● Inverted residue of binary division

p1(x) = xk (x15+x14+...+x2+x +1)

p2(x) = x16d(x)

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Communication Systemsdata link layer for the D channel error detection

● Checking for added or lost binary zeros● Thus cyclic Hamming codes implemented● Error detection for one, two and three bit error

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Communication Systemsnetwork layer for the D channel

● DSS1 protocol handels the call setup of the calling and called site

● Call destruction after finishing the session● Restaring and parking if required● Error handling

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Communication SystemsDSS1 layer 3 protocol

● Protocol Discriminator– part of the Layer 3 header information

– single byte (octet) that is usually set to a value of 00001000 (hexadecimal "08") - meaning Q.931 call maintenance

● Reference Value consists of either two or three bytes (octets)– BRI systems have a 7-bit Call Reference value (127

references)

– no particular end-to-end significance

– Either end can assign an arbitrary value

– used to associate messages with a particulary channel connection

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Communication SystemsDSS1 layer 3 protocol

● Message Type single byte (octet) that indicates what type of message is being sent/received

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Communication SystemsDSS1 layer 3 protocol – message types

● Message Type – four categories– Call Establishment– Call Information– Call Clearing– Miscellaneous

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Communication SystemsDSS1 layer 3 protocol – information elements

● Each type of message has Mandatory and Optional Information Elements, identified with single byte (octet)– Bearer Capability (identifies transport requirements of the

requested B-Channel)

– Cause (identifies reasons for disconnect or incomplete calls)

– Channel Identification (indentifies type and number of B-Channel(s) requested)

– Progress Indicator (Indicates status of outgoing call)

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Communication SystemsDSS1 layer 3 protocol – information elements

– Network Specific Facilities (Useful for North American PRI calls - identifies network type, Carrier ID, Carrier Service Type[WATS/SDN/ASDS,etc.])

– Calling Party Number (caller ID)

– Calling Party Number subaddress

– Called Party Number (destination number, type of number[unknown], numbering plan)

– Called Party Number subaddress

● When Information Elements consist of multiple octets, the following octet describes how many bytes (octets) are in the Information Element

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Communication Systemsliterature on telephony networks

● Have a nice holiday week!

● E. Pehl, Digitale und analoge Datenübertragung ● http://www.ks.uni-freiburg.de/php_termindetails.php?id=180● ...

http://www.ks.uni-freiburg.de/php_termindetails.php?id=180

1 | 54 communication systems 8 th lecture chair of communication systems department of applied...

Documents

communication systems

voice communication

traditional telephony

end systems

berlin slide

telephony provider

protocol slide

telephone device slide